There has been proposed a technique referred to as plasma doping in order to introduce a P-type impurity or an N-type impurity when forming a PN junction in a process for manufacturing a semiconductor such as a transistor.
This serves to expose a semiconductor substrate to a plasma containing an electrically active impurity to introduce an impurity into a surface portion of the semiconductor substrate with an energy of a plasma when doping the semiconductor substrate with the impurity.
Referring to a necessary impurity for a semiconductor, however, a purification or a quantitative control at an element and molecule level of the impurity is required in order to definitely give characteristics thereof, for example, electrically positive and negative characteristics.
In the case of some closing or semi-closing environment, more specifically, a vacuum chamber or an atmospheric pressure plasma, the plasma doping is carried out in a plasma treating mechanism in order to confine the plasma. For this reason, a thin film containing an impurity substance is deposited in a portion provided in contact with the plasma in a substance constituting the closing or semiclosing environment. The deposited substance comes in contact with the plasma, and is thus sputtered again by the energy of the plasma and is mixed into the plasma in some cases, and the plasma and the thin film are brought into a kind of stationary state by a synthesis of competing processes of the deposition and the sputtering. If the deposited substance is homogeneous to the impurity in the plasma, a balance is maintained by the deposition and a remixture. If the deposited substance is heterogeneous, a different kind of substance from the impurity contained in the plasma is mixed into the plasma. Consequently, the characteristic of the impurity contained in the plasma is confused. As a result, it is impossible to accurately control the characteristic and amount of the impurity to be doped into a semiconductor thin film. For this reason, the plasma doping method is used with difficulties in a manufacture of a semiconductor on which the characteristic greatly depends based on a very small difference in an amount.
The inventors have proposed that a vacuum chamber is sorted every type of an impurity to provide a special chamber in order to avoid the mixture (Patent Document 1). According to the Patent Document 1, for example, a right chamber is used as a P-type doping chamber 50 and a left chamber is used as an N-type doping chamber 60 in FIG. 5. A semiconductor substrate 130 is subjected to P-type patterning with a photoresist and P-type doping is performed in the chamber 50, and subsequently, the semiconductor substrate 130 is subjected to N-type patterning with the photoresist and N-type doping is performed in the chamber 60. By disposing the special plasma chamber for each desirable impurity, thus, it is possible to avoid the mixture of impurities having different characteristics. Therefore, it has been found that the plasma doping can be utilized in the manufacture of the semiconductor.
The inventors according to the invention further found that an amorphization of a silicon single crystal substrate through an inert gas, particularly, a He plasma plays a very important part (Patent Document 2). More specifically, even if a large amount of He is irradiated for a long period of time in a He ion implantation through a conventional beam line ion implantation, the silicon single crystal substrate cannot be amorphized. This has been a common sense of an ion implantation industry before 2004. When the plasma is used, however, a large amount of He (which is almost 100 times as large as the amount of an ion implantation) is irradiated in a unit time at a much lower energy as compared with the past common sense. For this reason, it has been found that the silicon single crystal is also amorphized.
Referring to the effect of the amorphization through He, an impurity is introduced subsequently to or simultaneously with the amorphization so that a profile in a direction of a depth of the impurity is changed to be preferable (Non-Patent Document 1). Consequently, it is possible to produce a special advantage linked to an enhancement in a uniformity and a control of the amount of the impurity.
It is obvious that a simple mixture of an impurity, for example, B2H6 into He does not produce a special effect. When the inventors made an experiment for plasma doping in the early stage, B2H6 was diluted into 5% with He and was thus used as described in Patent Document 4. However, it was impossible to produce the advantage of the amorphization.
Patent Document 1: Japanese Patent No. 2780419 Specification
Patent Document 2: Japanese Patent Application No. 2003-041123
Patent Document 3: Japanese Patent Application No. 2004-360122
Patent Document 4: Japanese Patent No. 2022204 Specification
Non-Patent Document 1: Y. Sasaki et al., “B2H6 Plasma Doping with In-situ He Pre-amorphyzation”, 2004 Symposia on VLSI Technology and Circuits
Patent Document 5: JP-A-2004-179592